Vibratory feeding, conveying or milling apparatus has been used in industry for many years. A basic type of vibratory feeder is shown in the Musschoot et al U.S. Pat. No. 3,089,582 wherein a two-mass exciter system is employed to impart vibratory feeding or conveying movement to a material-carrying member or trough. In the vibration generating apparatus shown in that patent, a constant speed motor carries a pair of eccentric weights, one on each end of the motor shaft, and the vibrational force generated with operation of the motor is transmitted through a spring system to the material carrying trough. In the apparatus shown in said patent, the amplitude of vibration is constant, hence a constant nonvariable feed is provided.
An improved variable rate vibratory feeder or conveyor is shown in Musschoot et al U.S. Pat. No. 3,358,815 wherein the electric motor carries at each end of its shaft a wheel-like member rotated with the shaft, with the wheel-like member being provided with a weight shiftable in an arcuate path from a first position near a fixed eccentric weight on each wheel to a second position opposite the fixed weight. The preferred system shown in the last-mentioned patent is one where the weight is a fluid, namely mercury, and thus readily susceptible to movement by varying the pressure. In some situations, mercury is considered as a pollutant and hence the mercury type of variable rate feeder has not been universally accepted.
In bowl-type milling apparatus, the vibratory movement is provided with a lead angle which will roll the material as it is being milled to a desired finish. One such device is shown in the U.S. Pat. No. 4,042,181 to Huber et al entitled "Lead Angle Controlling Mechanism" wherein one eccentric weight is fixed and a second eccentric weight is adjustable relative to the fixed eccentric weight so that in operation the resultant force has a particular lead angle that acts to roll the material being milled. A specific lead angle is employed for each product to maximize the roll for proper finishing. The apparatus required stops and bumpers and springs and worm gear devices to set the positions of the second eccentric weight relative to the first eccentric weight.